1. Field of the Invention
The present invention relates to a control system for an internal combustion engine that distributes a fuel mixture directly into a combustion chamber and distributes a stratified fuel mixture locally around a spark plug.
2. Description of Related Art
In the recent years, it is typical in the field of internal combustion engine to change a pressure of fuel that is sprayed directly into a combustion chamber of an internal combustion engine between stratified charge combustion and homogenized charge combustion in order to concentrate a fuel mixture locally around a spark plug during the stratified charge combustion and to promote atomization of a fuel mixture during the homogenized charge combustion. Such an internal combustion engine is known from, for example, Japanese Unexamined Patent Publication 2000-204954.
It is a recent tendency to install a NOx absorbing material in an exhaust system in consideration of an air-fuel ratio that is made lean due to the stratified charge combustion. The NOx absorbing material absorbs NOx in an oxygen excess exhaust gas and purge the NOx as the oxygen concentration of the exhaust gas lowers. Under ordinary stratified charge combustion in which an air-fuel ratio is lean, the NOx absorbing material absorbs NOx in an exhaust gas. When the NOx absorbing material absorbs more than a specified amount of NOx, the fuel combustion is altered in combustion mode from the stratified charge combustion to the homogenized charge combustion in which an air-fuel ratio is rich for a comparatively short period of time so as to lower the oxygen concentration of an exhaust gas. As a result of this, the NOx absorbing material is caused to purge NOx so that the purged NOx is reduced or deoxidized with unburned HC and CO that are increasingly produced through the homogenized charge combustion. Thereafter, when the amount of NOx absorption in the NOx absorbing material is decreased to a satisfactory level, the stratified charge combustion resumes. Accordingly, when the homogenized charge combustion is performed in order to cause the NOx absorbing material to purge NOx, the pressure of a fuel spray is increased to a level meeting a combustion demand. When the stratified charge combustion resumes, the pressure of a fuel spray is decreased so as to meet the stratified charge combustion.
However, when the combustion mode returns to the stratified charge combustion from the homogenized charge combustion after the NOx absorbing material completely purges NOx, a change in the pressure of a fuel spray occurs with a time delay. In this event, the fuel is sprayed with a fuel spray penetration force higher than a primary fuel splay penetration force for the stratified charge combustion, as a result of which a fuel mixture is hard to concentrate locally around the spark plug upon an alteration to the stratified charge combustion from the homogenized charge combustion, so that a satisfactory stratified charge combustion is hardly realized.
It is an object of the present invention to provide a control system for an internal combustion engine that prevents an occurrence of unsatisfactory stratified charge combustion of a fuel mixture when a combustion mode alteration to stratified charge combustion from homogeneous charge combustion for causing a NOx absorbing material in an exhaust system to purge NOx.
The aforesaid object of the present invention is accomplished by an engine control system for controlling an internal combustion engine that comprises fuel injection means for spraying fuel directly into a combustion chamber, fuel injection pressure control means for controlling a pressure of fuel that the fuel injection means sprays, intake air regulation means disposed in an intake line for regulating an amount of intake air that is introduced into the intake line, NOx absorption means disposed in an exhaust line for absorbing NOx in an exhaust gas at a high oxygen concentration and releasing the NOx in an exhaust gas at a low oxygen concentration, speed monitoring means for monitoring an engine speed of rotation of the internal combustion engine, and control means for controlling the fuel injection means and the fuel injection pressure control means on the basis of the engine speed of rotation.
The control means controls the fuel injection means on the basis of the engine speed of rotation so as to perform homogeneous charge combustion by spraying fuel into the combustion chamber in an intake stroke of a cylinder when the engine speed of rotation is higher than a specified speed of rotation and to perform stratified charge combustion by spraying fuel into the combustion chamber in a compression stroke of the cylinder when the engine speed of rotation is lower than the specified speed of rotation, controls the fuel injection means and/or the intake air regulation means to change an air-fuel ratio during execution of the homogeneous charge combustion so as to provide a rich state wherein an oxygen concentration of an exhaust gas is smaller than that when an air-fuel ratio is equal or nearly equal to a stoichiometric air-fuel ratio, controls the fuel injection pressure control means to control the pressure of fuel according to the engine speed of rotation when the stratified charge combustion is executed and to bring the pressure of fuel to a fixed pressure higher than a pressure of fuel with which the stratified charge combustion is executed when the homogeneous charge combustion is executed, estimates an amount of NOx absorbed in the NOx absorption means; controlling the fuel injection means so as to spray fuel suitably for the homogeneous charge combustion for a specified period of time when it is judged that the amount of NOx has reached a specified amount of NOx during execution of the stratified charge combustion; and controlling the fuel injection pressure control means so as to bring the pressure of fuel provided during execution of the homogeneous charge combustion resulting from the judgment lower than the pressure of fuel provided during execution of the homogeneous charge combustion on the basis of the engine speed of rotation.
According to the engine control system, during execution of the homogeneous charge combustion in order to purge NOx from the NOx absorption material, a change of a pressure of fuel to a demand pressure of fuel of the homogeneous charge combustion is restricted. As the result of the restriction of the pressure change to the demand pressure of fuel of the homogeneous charge combustion, while the NOx absorption material purges NOx into an exhaust gas so as to reduce the NOx and to purify the exhaust gas during execution of the homogeneous charge combustion, a fuel mixture is prevented from becoming apt to fall into the difficulty that the fuel mixture is hard to concentrate locally around a spark plug due to a higher fuel spray penetration force than a primary fuel spray penetration force for the stratified charge combustion that is caused resulting from a time delay in changing the pressure of fuel even upon return to the stratified charge combustion from the homogeneous charge combustion. This causes the stratified charge combustion to be achieved satisfactorily upon resumption of the stratified charge combustion from the homogeneous charge combustion for NOx purge.
The control means may control the fuel injection means during execution of the homogeneous charge combustion resulting from the judgment so as to correct a fuel injection timing at which the fuel injection means sprays fuel according to a pressure difference of the pressure of fuel provided during execution of the homogeneous charge combustion resulting from the judgment from the pressure of fuel provided during execution of the homogeneous charge combustion on the basis of the engine speed of rotation from. This correction of a fuel injection timing prevents deterioration of the combustibility of fuel due to the restriction of the change to the demand pressure of fuel of the homogeneous charge combustion.
The engine control system may further comprise an exhaust gas recirculation line interconnecting an intake line and an exhaust line, and exhaust gas recurculation control means. In this engine control system, the control means controls the exhaust gas recurculation control means so as to bring an amount of exhaust gas that is recirculated during execution of the homogeneous charge combustion resulting from the judgment smaller than an amount of exhaust gas that is recirculated during execution of the homogeneous charge combustion on the basis of the engine speed of rotation. The reduction in the amount of exhaust gas recirculation prevents deterioration of the combustibility of fuel due to the restriction of the change to the demand pressure of fuel of the homogeneous charge combustion.